Twitter announced this morning that its new Lead Generation cards, which had been in limited beta since May, are now available for all advertisers on Twitter — including small self-serve advertisers.

And they’re generating huge engagement — at least in early tests.

Lead Generation cards are pretty much what they sound like. Tweets with lead generation enabled include a simple way for interested consumers to give their e-mail address to the advertiser, right on Twitter, without leaving the web page or app. Then the advertiser can follow up and, perhaps, close the sale, or at least nurture a customer.

At least one campaign for Rock/Creek outdoor gear company saw massive engagement in the initial trial period, with a 4.6 percent engagement rate and over 1,700 new e-mail contacts. While 4.6 percent may not sound massive, it’s big in online marketing, where you could see clickthrough rates of less than 1 percent on some marketing campaigns. For comparison, promoted tweets on desktop — where you can see higher clickthrough than mobile — are perhaps 1 percent to 3 percent for promoted tweets.

Lead Generation cards, at least initially, are seeing higher engagement — perhaps due to the fact that you don’t need to leave Twitter to engage with them.

And because the targeting can be very, very tight.

“I was thrilled with the cost per new email we saw,” Mark McKnight from Rock Creek told Twitter. “Because we’re able to precisely target interests and @usernames, we also know that these people are highly likely to be interested in our products.”

The Lead Generation cards take “just minutes” to set up, Twitter says, and the resulting leads can be downloaded in a CSV file, or streamed directly into your customer relationship management system.